This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Chronic Fatigue Syndrome (CFS) is a diabling condition characterized by chronic fatigue of at least 6 months duration an multiple somatic complaints, including memory loss, trouble concentrating and dizziness. The lack of a biological marker as well failure of any reproducible double-blind, placebo-control studies to demonstrate any therapeutic response makes CFS an extremely difficult disease to diagnose and treat. I hypothesize that some forms of CFS are due to inadequate cerebral blood flow resulting in the symptomatology seen in chronic fatigue symdrome including fatigue, dizziness and congitive impairment. This impaired cerebral blood flow would be visible as filling defects on brain SPECT scans. Appropriate treatment for CFS, therefore, would include agents which selectively increase cerebral blood flow. We propose that a directed approach to increase cerebral blood flow would be to use sildanifil (Viagra), an inhibitor of the type 5 cyclic guanosine monophosphate (cGMP) phosphodiesterase. Our objectives, therefore, are to test the following hypothesis: Patients with CFS treated with sildenafil are expected to have increased cerebral blood flow with an improvement of their fatigue and functional status as well as SPECT scan abnormalities. This hypothesis will be studied as follows: Patients will receive either sildenafil or placebo in a randomized, double-blind, placebo-controlled study for 6 weeks of active drug. Functional status, fatigue index, and brain SPECT scans will be monitored at baseline andd after each treatment period.